Most electrochemical cells used in electrical storage batteries comprise an electrolyte interposed between and in contact with an anode and a cathode. The anode comprises an active material that is readily oxidized, and the cathode comprises an active material that is readily reduced. During discharge, the anode active material is oxidized and the cathode active material is reduced, so that electrons flow from the anode through an external load to the cathode, and ions flow through the electrolyte between the electrodes.
Many electrochemical cells used in electrical storage batteries also include a separator between the anode and the cathode to prevent reactants and reaction products present at one electrode from reacting and/or interfering with reactions at the other electrode. In modern batteries, anodes and cathodes may be in close physical proximity, and thus, to be effective, a battery separator must be electronically insulating, and remain so during the life of the battery to avoid battery self-discharge via internal shorting between the electrodes. In addition, a battery separator must remain a sufficiently good ionic conductor of ions, to avoid excessive separator resistance that substantially lowers the discharge voltage, while at the same time remaining a barrier to diffusion of deleterious reactants and reactant products to the opposite electrode.
Electrical storage batteries are classified as either “primary” or “secondary” batteries. Primary batteries involve at least one irreversible electrode reaction and may not be recharged with useful charge efficiency by applying a reverse voltage. Secondary batteries involve relatively reversible electrode reactions and may be recharged over numerous charge-discharge cycles with an acceptable loss of charge capacity. Since the separator must survive repeated charge-discharge cycles, separator requirements for secondary batteries tend to be more demanding.
For many secondary batteries comprising a highly oxidative cathode, a highly reducing anode, and an alkaline electrolyte, separator requirements are particularly stringent. The separator must be chemically stable in a strongly alkaline solution, resist oxidation in contact with the highly oxidizing cathode, and resist reduction in contact with the highly reducing anode. Since ions, especially metal oxide ions, from the electrodes may often be somewhat soluble in alkaline solutions and be chemically reduced to metal on separator surfaces, the separator must also inhibit transport and/or chemical reduction of metal ions. Otherwise, a buildup of metal deposits within separator pores may increase the separator resistance in the short term, and in the long term, may cause shorting failure due to formation of a continuous metal path through the separator.
In addition, because of the strong tendency of many anodes to form dendrites during charging, the separator must suppress dendritic growth and/or physically resist dendrite penetration to avoid failure due to an electrolyte leak or formation of a dendritic short between the electrodes. A related issue with anodes is shape change, in which the central part of the electrode tends to thicken during charge-discharge cycling. The causes of shape change are complicated and not well understood but may involve differentials in the current distribution and solution mass transport along the electrode surface. The separator preferably mitigates electrode shape change by exhibiting uniform and stable ionic conductivity and ionic transport properties.
Zinc alkaline and particularly zinc-silver rechargeable batteries are used in many applications because of their high power density. These batteries possess one of the highest gravimetric and volumetric energy densities of commercially available batteries. Additionally, traditional zinc batteries possess low self-discharge rates as well as high current discharges upon demand.
However, traditional zinc batteries with traditional separators suffer a number of limitations. For example, these batteries suffer from a sharp decline in capacity with usage that results in a short charge/discharge cycle life, e.g., lasting less than 50 cycles when subjected to field conditions with infrequent cycling, short overall service life, or both. This sharp reduction in capacity is predominantly caused by secondary chemical reactions that occur in zinc battery cells. These secondary chemical reactions may cause the degradation of the electrolyte, a change of the scope of the anode electrode due to excessive zinc solubility in an aqueous electrolyte, a degradation of the electrode separator via silver migration and plating, and premature localized shorts due to the formation of dendrites on the zinc electrode. It is also noted that these deleterious secondary reactions may be brought about by overcharging the battery during recharge.
For improved performance, separators in zinc alkaline and zinc silver batteries must satisfy many of the numerous and often conflicting requirements listed above. Specifically, zinc battery separators are required to be resistant to electrochemical oxidation and silver ion transport from the cathode and resistant to electrochemical reduction and dendrite penetration from the anode.
Cellulose separators, in the form of regenerated cellulose separators, have often been used in zinc-based batteries, such as for example, zinc alkaline and zinc silver batteries, because of their ability to allow negative OH− ions through the membrane in high alkaline environments with low electrical resistivity while resisting the passage of metal ions. Over time, however, cellulose separators decompose chemically in alkaline electrolytes, which limits the useful life of the battery. They are also subject to chemical oxidation by soluble silver ions and electrochemical oxidation in contact with silver electrodes. Furthermore, some cellulose separators may exhibit low mechanical strength and poor resistance to penetration by dendrites.
To solve some of the problems caused by traditional separators, new separator materials have been developed.